1. Field of the Invention
The present invention relates to a white light emitting device, and more particularly, to a white light emitting device, which can achieve high emission efficiency by use of a blue light emitting diode (LED) having a plurality of active layers that generate different center wavelengths.
2. Description of the Related Art
Recently, much attention has been drawn to light emitting diodes (LEDs) as light sources for backlight units (BLUs) employed in liquid crystal displays (LCDs) for lap top computers, monitors, mobile phones, TVs and the like. Cold cathode fluorescent lamps (CCFLs) have been in use as white light sources for BLUs. However, of late, white light source modules employing LEDs have captured attention due to their advantages such as better color representation and lower power consumption.
To implement a white LED light source, a white light emitting device may employ LEDs of the three primary colors, blue (B), green (G) and red (R) in one package. Such a white light emitting device may ensure relatively high color representation and the overall control of output light by adjusting the quantity of light from the R, G and B LEDs. However, the R, G and B LEDs, spaced apart from each other, may undermine color uniformity. Particularly, the R LED is sensitive to the temperature and has a light efficiency which is susceptible to degradation by heat, resulting in the unstable uniformity of white light. Moreover, to produce white light, at least one set of R, G, and B LED chips is required in a unit area. This entails a complicated circuit configuration to drive and control the LED of each color, thus leading to higher manufacturing costs for packages.
Alternatively, a white light emitting device may be implemented by applying phosphors, emitting red, blue and green light, to ultraviolet (UV) LED chips at predetermined ratios. However, this white LED fails to produce high-luminance white light because the emission efficiency of phosphors emitting red light is low in spite of the high excitation energy of UV light emitted from the UV LED chip. Moreover, the three colors of the phosphors, which are mixed at a specific composition ratio, need to be properly matched so as to produce white light. This requires sensitive and complicated processes in preparing the phosphors.
Alternatively, a white light emitting device may also be implemented by using the combination of blue LEDs and yellow phosphors excited by blue light from the blue LEDs. This white light emitting device ensures simple circuit configuration and low costs. FIG. 1 illustrates the emission spectrum of a related art white LED manufactured by using the combination of GaN-based blue LEDs and YAG-based yellow phosphors. Referring to FIG. 1, the intensity of light is high in both the blue and yellow wavelength regions. However, the wavelength of the LED in a short wavelength range, which does not match the excitation wavelength of the phosphors, undermines the excitation efficiency of the phosphors, thus lowering the luminance level thereof.
Therefore, low-cost, high quality white LEDs are required, which can achieve high and stable color uniformity and enhanced color reproducibility.